1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses having a large relative aperture up to 1.4 in F-number and a high zoom ratio up to 15, while still maintaining good optical performance throughout the entire range of magnification, suited to photographic cameras, video cameras, etc.
2. Description of the Related Art
In the photographic cameras and video cameras, there has been a growing demand for large relative aperture, high range zoom lenses of high optical performance.
Of these, the video camera, because of its image pickup element having a relatively low sensitivity, has been calling for as large an increase in the relative aperture of the zoom lens as possible.
Besides this, in the color camera using the single tube type color image pickup tube, or other image pickup device such as CCD or like image pickup plate, in combination with a stripe filter, for the purpose of preventing mixture of colors, it is required to construct the entirety of the optical system in telecentric form so that the light beam entering the aforesaid stripe filter becomes as perpendicularly incident as possible.
Again, the CCD or MOS is now widely used as the image pickup device in the video camera. The front surface of its cover glass and the image receiving surface have, in general case, a high reflectance. For this reason, the light reflected from these surfaces scatters from the lens surfaces and on the inside of the lens barrel, arriving again at the image pickup device. The so-called ghost and flare arise from such interreflections.
Particularly when an ND filter or the like is used adjacent to the stop of the photographic lens, the interreflections between the surfaces of this filter and the stripe filter often lead to ghost images or flare spots on the image pickup device, causing the optical performance to lower extremely.
Thus, for the photographic lens to be used in the video camera, the requirement of getting good optical performance without suffering from production of harmful light rays from which the ghost and flare arise must be fulfilled.
In this respect, a proposal for getting a zoom lens of high grade imagery is made in U.S. patent application Ser. No. 223,108 filed on July 22, 1988.
In this proposal, concerning each lens surface on the image side of the stop, ray tracing is done on the assumption that the returning light ray from the image plane is reflected at that surface. The radii of curvatures of all these lens surfaces are so determined by using the bending technique that the reflected light, even if coming to focus, leads to an image toward the photographic lens away from the prescribed image plane. In such a way, the harmful light rays are removed. The reason why the lens surfaces on the object side of the stop are not considered here is that with the stop at full open aperture, because there exist light rays of various angles, the harmful light rays are distributed uniformly over the entire picture area, so that their influence is not very conspicuous, giving no problem on the actual practice. When the stop is closed down, however, the influence becomes serious. Therefore, particular attention is paid on the surfaces in rear of the stop which contribute to an interreflection when the stop is closed down.
By the way, when the stop is closed down, the diaphragm blades come to produce a considerable influence on the diffracting phenomenon, causing the image quality to deteriorate. So, to avert this phenomenon, the acceptable minimum size of aperture opening has to be set at a relatively large value. It is known to provide a technique of using an ND filter for fulfilling the light reduction function when a smaller aperture size than this minimum size is necessary.
The use of this filter, however, gives rise to a new problem. For example, as this filter is put in a space constituting part of an afocal system, a light beam coming backward from the image pickup device is reflected again at this filter. The reflected light beam is again focused, forming a ghost image near the image pickup device. And, a technique of preventing this ghost is proposed in U.S. Pat. No. 4,892,397. The present invention falls in improvements of this zoom lens.
Meanwhile, as the zoom lens suited to the video camera, mention may be made of U.S. Pat. Nos. 4,618,219, 4,621,905, 4,659,187, 4,653,874 and 4,518,228. There are other ones in Japanese laid-Open Patent Applications Nos. Sho 59-222807, Sho 60-260912 and Sho 61-20291 and U.S. Pat. No. 4,812,024.